A thin film EL element usually has a structure as shown in FIG. 1. It is made up of a transparent substrate 1, and a transparent conducting film 2, a first dielectric layer 3, a luminescent layer 4, and a second dielectric layer 5, which are formed on top of the other on the substrate. The transparent conducting film and the second dielectric layer 5 are provided with electrodes 6a and 6b, respectively. Upon application of a strong electric field across the terminals 6a and 6b, the luminescent layer 4 emits light which emanates through the transparent substrate 1.
In the foregoing thin film EL element having two dielectric layers, the luminescent layer 4 is made of a luminescent host material doped with an impurity for the luminescent center. A strong electric field applied to the luminescent layer 4 excites the electron energy level of the luminescent center. When the excited state returns to the ground state, the conversion of energy into light takes place. The result is electroluminescence. The light emitted by electroluminescence has a specific wavelength which depends on the luminescent center.
For electroluminescence to take place, it is necessary that a strong electric field be applied to the luminescent layer and the luminescent host material have a broad band gap. A luminescent host material having a narrow band gap does not withstand the strong electric field applied to the luminescent layer 4, but permits an electric current to flow through it. This prevents the application of a strong electric field to the luminescent layer. This is the reason why the conventional luminescent host material was selected from the II-VI compounds, such as ZnS, SrS, CaS, and ZnSe, which have a broad band gap, and the luminescent center was Mn or Eu. Mn-containing ZnS (ZnS:Mn) emits yellowish-orange light and Eu-containing CaS (CaS:Eu) emits red light.
The ZnS:Mn used for the above-mentioned luminescent layer 4 takes on a crystalline structure, with the impurity present as shown in FIG. 2. That is, the Mn atom (of a divalent transition metal) takes the lattice point where the divalent Zn atom of ZnS (II-VI compound) should be. This results in a stable EL element with high luminance.
For the emission of light of different colors, attempts have been made to use a variety of elements as the impurity for the luminescent center. They include, for example, Tb, Tm, Sm, and Ce. When ZnS is doped with Tb, Tm, or Sm, the resulting ZnS:Tb, ZnS:Tm, or ZnS:Sm emits green light, blue light, or reddish-orange light, respectively. When SrS is doped with Ce, the resulting SrS:Ce emits bluish-green light.
The II-VI compound as the luminescent host material is electrically neutral and hence remains stable in its compound form, because the II Group element has a valence of 2 and the VI Group element has a valence of -2. When the luminescent host material is doped with an impurity for the luminescent center, the atoms of the former are partly replaced by the atoms of the latter. In the case where the impurity is a divalent element such as Mn and Eu, the luminescent layer remains electrically neutral and hence stable. This is not true of the case in which the impurity for the luminescent center is a trivalent element such as Tb and Tm. In such a case, the luminescent layer loses the electrical neutrality, giving rise to the vacant lattice point, crystal transition, and crystal strain. Such crystal imperfections prevent the energy applied to the luminescent layer to be effectively transmitted to the impurity for the luminescent center, and hence prevents the light emission with high luminance at a low voltage.
In the case where the luminescent layer 4 is made of ZnS:Tb, the impurity (Tb) exists in the crystal as shown in FIG. 3. Since Tb is trivalent, ZnS:Tb as a whole is not electrically neutral and hence does not glow with sufficient luminance. In order to overcome this disadvantage due to a trivalent impurity, it has been proposed to use Tb together with a charge compensating element such as F, thereby forming ZnS:Tb,F.sub.x (x=1 to 3), which glows brightly. The ZnS:Tb,F.sub.x takes on the crystalline structure in which the impurity exists as shown in FIGS. 4 and 5. The existence of F atoms in the crystal makes the luminescent layer unstable and hence poses a problem associated with its reliability (operational life).
A possible alternative is to use a III-V compound, with the trivalent element therein partly replaced by a trivalent impurity for the luminescent center. A disadvantage of this doped compound is that it is necessary to apply a strong electric field (about 1 MV/cm) to excite the luminescent center. Any material to withstand such a strong electric field should have a band gap greater than about 3 eV. However, ordinary III-V compounds (such as InP, GaP, and GaAs) lack such a great band gap and hence are unsuitable for the fluorescent layer.
In addition, the conventional thin film EL element mentioned above has a luminescence efficiency as low as about 0.1%. One reason for the low luminescent efficiency is an insufficiency of excitons. It is considered that excitons take part in luminescence of the luminescent layer 4. It is also considered that luminescence of the conventional EL element is caused by excitons present within or near the impurity for the luminescent center.